Sintering is the welding together of small particles by heating such particles without melting them at temperatures below the temperature at which they flow completely; the particles are fused together to form a relatively strong mass, but the mass as a whole does not melt. Sintering is used in powder metallurgy but has application with ceramics, cellulose derivatives and polytetrafluoroethylene resin compositions. The prior art methods for producing sintered articles generally comprises the steps of introducing finely divided powder into a mold, compressing the powder to a preform of a desired shape, heating the preform at a sintering temperature, and then cooling the sintered article. When sintered articles with porous structures are desired, volatile materials are generally mixed with the preform; upon sintering the volatile material evaporates or escapes leaving voids in the sintered product.
Examples of these methods are disclosed in U.S. Pat. Nos. 1,873,223 (sintering a mixture of metal particles and a volatile lubricant to form porous bearings or similar articles), 1,742,515 and 1,934,383 (sintering ceramic particles in the presence of a volatile component to form porous bricks), 2,085,047 (forming a porous article by heating a mixture of a derivative of cellulose and a salt or other compound that is insoluble in the mixture at a temperature high enough to decompose the salt or other compound but not high enough to melt the cellulose derivative), 2,252,277 (heating a mixture of "Bakelite" and a pore forming substance at a high enough temperature to evacuate the latter leaving a porous structure), and 2,356,076 (forming a porous carbon plate by heating a mixture of green carbon fluor and particles of a volatile substance at a temperature that causes the latter to change to a gas and escape leaving cavities conforming to the original particles).
U.S. Nos. 2,400,091; 2,413,498; 2,456,262; 2,710,991; and 2,985,918 disclose various techniques for shaping sintered articles of polytetrafluoroethylene. U.S. Pat. No. 2,400,091 discloses a process for producing a porous article by the steps of heating polytetrafluoroethylene at a temperature in the range of 327.degree. C. to 500.degree. C., cooling the polymer to a temperature below 327.degree. C., subdividing the cooled polymer to a particle size less than about 20 mesh, forming the finely divided polymer in the shape of the desired article by pressing it in a mold at a temperature below 327.degree. C., heating the shaped polymer at a temperature between 327.degree. C. and 500.degree. C. until it is heated throughout to said temperature, and cooling the resulting article. U.S. Pat. No. 2,413,498 discloses a molding process that includes the steps of mixing polytetrafluoroethylene and a heat-decomposable film-forming material and then removing the film-forming material at a temperature sufficiently high to flash off the film-forming material, and continuing heating until the polytetrafluoroethylene coalesces in the form of desired shaped articles such as rods, tubes, and unsupported forms. U.S. Pat. No. 2,456,262 discloses a process of fabricating polytetrafluoroethylene articles by the steps of subjecting finely divided polytetrafluoroethylene to a pressure in a die to form a shaped body, releasing the pressure and removing the shaped body from the die, heating the shaped body at a temperature above 327.degree. C. until it is sintered, and cooling the shaped body to a temperature below 250.degree. C. under pressure in a die having the shape of the desired finished article. U.S. Pat. No. 2,710,991 discloses a method of producing thin sheet-like articles from polytetrafluoroethylene which includes the steps of molding preforms at substantially room temperature, releasing the molding pressure, heating the preforms to a sintering temperature and then cooling the sintered article. The preforms are molded from the polymer in dry powdered form by spreading a plurality of charges of the dry powder one above the other in the mold, the charges being separated by flexible separator sheets, and consolidating the charges by the application of pressure. U.S. Pat. No. 2,985,918 discloses a process for forming shaped lengths of polytetrafluoroethylene having a cellular structure that includes the steps of extruding through a die a paste composition of finely divided polytetrafluoroethylene and finely divided methyl methacrylate polymer intimately mixed with a volatile organic lubricant and thereafter heating the resultant shaped article to sinter the polytetrafluoroethylene and to decompose and volatilize the acrylic polymer to form minute interconnecting pores throughout the article.
The prior art methods teach the use of conventional heating techniques. The term "conventional heating", and cognate terms such as "conventional heat", are used herein to mean heating by conduction or convection or a combination thereof using a conventional heating source, such as an open flame or a heating coil, with the heat being transferred to the article being heated by a solid, liquid or gaseous (e.g., air) medium. Conventional heating is not used herein to include "dielectric heating" which is defined hereafter.
Effective sintering of polymeric preforms to attain desirable levels of strength and porosity for the sintered articles produced therefrom requires sufficient fusing of the individual polymeric particles both at the surface and in the interior of the article without such particles completely melting. Porous polymeric preforms exhibit relatively poor heat transfer characteristics (partially due to their porous structure) and, consequently, effective sintering of such materials can be attained with conventional heating only at the cost of lengthy and carefully monitored heating cycles. Insufficient heating results in sintered articles with poor fusion in their interiors and, consequently, inadequate strength. Overheating results in sintered articles with overfusion on their exterior surfaces and, consequently, inadequate porosity. The proposal of admixing volatile components to compensate for the lengthy carefully monitored heating cycles results in additional preform preparation procedures and, in some instances, procedures for removing excess volatiles subsequent to sintering. Additionally, careful monitoring and control of the heating cycle is still required to avoid cracking or splitting due to excessively rapid or uncontrolled evaporation of the volatile component.